A small amount of dissolved foreign fluids is practically always present in cryogenic liquids for technical applications of the purity usually used for this purpose. Thus, commercially available liquid hydrogen contains, e.g., dissolved oxygen, nitrogen and other substances.
As long as these impurities are present in the cryogenic liquid uniformly at low concentration and in the liquid state, they are usually unproblematic and no attention needs to be paid to them.
However, problems and hazards may develop when local enrichments of these foreign substances are formed. Such enrichments or deposits develop primarily if the foreign fluids have higher melting and boiling points and a higher specific gravity than the cryogenic carrier fluid.
During normal operation, the foreign fluids are transported practically into all areas of the cryosystem to which there is flow, i.e., into lines and various types of assembly units, as a consequence of the flow conditions.
During an interruption in the operation, cryogenic liquid initially remains in areas of the pipelines and assembly units. This cryogenic liquid cannot flow back into the tank under the action of gravity for construction-and function-related reasons. However, the cryogenic liquid is the first to evaporate due to heat supply from the environment and as a consequence of its lower boiling point, and it escapes from the said areas, preferably in the direction of the tank. The higher-melting and higher-boiling foreign substances remain behind in the liquid or even solid, frozen form.
Increasing amounts of foreign substances can accumulate in the said areas as the number of phases of operation and interruptions in operation increase, until critical concentrations develop locally. Explosions may occur in the extreme case at the time of the subsequent start-up of the system, i.e., at the time of the first contact of the cryogenic liquid flowing in with the concentrated foreign substances. There is a risk of explosion especially on contact of combustible cryogenic liquids with accumulations of oxygen. The oxyhydrogen gas reaction of hydrogen and oxygen is generally known in this connection.
The fact that frozen impurities reduce or block flow cross sections or impede or block the movement of moving functional elements, such as valve pistons, slide valves, pump impellers, etc., has less devastating, but still critical consequences.